The research emphasis is to characterize the genetic and molecular epidemiology, and natural history of Chlamydia trachomatis ocular infections by comparing, longitudinally, the diversity of ocular chlamydiae at the DNA level in trachoma endemic areas. Specific epidemiologic tools will be developed during Phase I for this objective. They will consist of a sensitive, non-radiolabeled DNA probe, serovar-specific (major outer membrane protein (MOMP)-based) oligonucleotide probes, a combined PCR/DNA probe assay to detect chlamydial DNA in specimens from the nasopharynx, and respiratory, intestinal, and urogenital tracts, and a genotyping technique for detecting mutations in the MOMP gene; the MOMP is considered the major virulence determinant of this organism. These methods will be utilized in a trachoma endemic area to prospectively evaluate: 2) the diversity of chlamydia serovariants that are responsible for infection; 2) the specific genotypes of these serovariants (to determine subtle fluctuations occurring below the level of serovar that may identify additional antigenic subtypes that are responsible for ocular infection); 3) the emergence of new subtypes based on mutations in the MOMP gene; 4) the mechanisms of antigenic variation of the MOMP; 5) the potential reservoirs of chlamydiae in the human host; and 6) the transmission patterns of this organism. This research is important for rational vaccine development.